To be commercially profitable, gypsum products, such as wallboard, are typically manufactured by continuous high speed processes. Manufacturers mine and transport gypsum to a mill in order to dry it, crush/grind it and calcine it to yield stucco. The reaction for the calcinations process is characterized by the following equation:CaSO4.2H2O+heat→CaSO4.½H2O+1½H2OThis equation shows that calcium sulfate dihydrate plus heat yields calcium sulfate hemihydrate (stucco) plus water vapor. This process is conducted in a calciner, of which there are several types known in the art. The stucco can contain one of two forms of calcium sulfate hemihydrate: the α-hemihydrate form and the β-hemihydrate form. These two types of stucco are often produced by different means of calcination. While the β-hemihydrate form is normally used due to its lower cost, either type of calcium sulfate hemihydrate is suitable for use.
Calcined gypsum (stucco) has the valuable property of being chemically reactive with water, and will “set” rather quickly when the two are mixed together. This setting reaction reverses the above-described stucco chemical reaction performed during the calcination step. The reaction proceeds according to the following equation:CaSO4.½H2O+1½H2O→CaSO4.2H2O+heatIn this reaction, the calcium sulfate hemihydrate is rehydrated to its dihydrate state over a fairly short period of time. The actual time required for this setting reaction generally depends upon the type of calciner employed and the type of gypsum rock that is used. The reaction time can be controlled to a certain extent by the use of additives such as accelerators and retarders.
While conventional gypsum products have many advantages, it has also long been desired to reduce the cost of manufacturing gypsum wallboard. One method of reducing the cost of wallboard has been to reduce the amount of water used in the manufacturing of the wallboard. Reduction in water reduces the amount of free water left in the wallboard after the setting reaction. A lower amount of free water left in the wallboard results in less drying energy being expended to remove the free water, which in turn saves energy costs associated with drying wallboard (i.e., the fuel cost associated with operating a kiln to dry the wallboard). However, the reduction of water negatively impacts the quality of the board produced (i.e., the strength paper-to-core bond and the compressive strength of the board may be decreased), because the reduction of water usually results in some of the calcium sulfate hemihydrate not being rehydrated to its dihydrate state. Wallboard gets its strength from the formation of crystals of calcium sulfate dihydrate during this rehydration process. The adverse effect on wallboard quality from reduced water levels prevents manufacturers from further cutting manufacturing costs by reducing the amount of water used in the slurry composition.
Moreover, lightweight wallboard is often produced by incorporating excess foam into calcined gypsum slurries, which in turn yield corresponding permanent voids in the product when the set gypsum is formed. Significant concentrations of foaming agents have to be employed to produce the desired concentration of voids in the set gypsum, in order to obtain a product of desired density because the aqueous foams employed are inherently unstable. The instability of the foaming agents leads to many of the bubbles coalescing and escaping the slurry before the set gypsum forms and thus, a significant amount of foaming agent must be used to get the desired lower density. The significant amounts of foaming agents used further increases material costs in the production of wallboard. Moreover, other materials must be used to increase the strength of lightweight wallboard, such as natural polymers (e.g., starch) which further increases the cost of manufacturing gypsum wallboard.
It is known in the art that sodium trimetaphosphate (“STMP”) can be utilized in the wallboard manufacturing process to increase the quality of wallboard. While STMP has this effect, it should be noted that the use of STMP in any wallboard manufacturing process is dependent on many factors. For example, STMP is ineffective in high pH level environments (i.e., greater than 10). Some manufacturing plants have to create slurries with high pH levels due to the type of gypsum the plant is using and in those environments, STMP has no effect. In all other environments, STMP can be used to increase the core quality of wallboard and allow manufacturers to reduce the amount of natural polymer (i.e., starch) used to produce wallboard of suitable quality. STMP is a combination of earth metals and is known by the chemical equation (NaPO3)3. In its solid form it is a salt, but when dissolved in an aqueous solution, it is present as a trimetaphosphate ion. It is believed that the addition of STMP to the slurry promotes long crystal growth. Such crystals add the most strength to the core of the wallboard and also serve to form stronger bonds between the core and paper. While in theory, STMP can be used by manufacturers to decrease costs by increasing the core quality of wallboard, it has been discovered in practice that the current methods of adding STMP leads to further problems and delays.
In known manufacturing processes for gypsum wallboard, a slurry is formed by premixing dry and wet ingredients in a mixing apparatus, such as a pin mixer. The dry ingredients can include, but are not limited to, any combination of calcium sulfate hemihydrate (stucco), fiberglass, accelerator, and in some cases natural polymer (i.e., starch). The wet ingredients can be made of many components, including but not limited to, a mixture of water, paper pulp, potash, and in some cases natural polymer (hereinafter, collectively referred to as a “pulp solution”). The pulp solution provides a significant portion of the water that forms the gypsum slurry of the core composition of the wallboard. The dry ingredients and the pulp solution contain the basic chemical components of a piece of wallboard and conventional methods of preparing such wallboards are well known to those skilled in the art. For example, the dry ingredients and pulp solution can be mixed together in a pin mixer. In this manner, the dry ingredients and pulp solution create a fluid mixture or “slurry.”
The slurry is discharged from the mixer through the mixer's outlet chute or “boot” which spreads the slurry on a moving, continuous bottom sheet of paper. A moving, continuous top sheet of paper is placed on the slurry and the bottom paper sheet, so that the slurry is positioned in between the top and bottom sheets of paper to form the board. The board can then pass through a forming station which forms the wallboard to the desired thickness and width. The board then travels along a belt line for several minutes, during which time the rehydration reaction occurs and the board stiffens. The boards are then cut into a desired length and then fed into a large, continuous kiln for drying. During drying, the excess water (free water) is evaporated from the gypsum core while the chemically bound water is retained in the newly formed gypsum crystals.
In such manufacturing processes, it is known to add desired quantities of STMP (in either its dry form or in solution) to the dry ingredients or to the wet ingredients. In practice, the STMP added in either of these manners initially dissolve in the pulp solution. However, during the mixing of the dry ingredients with the pulp solution to form the slurry, a portion of the STMP precipitates out as phosphates due to cationic contents in the pulp solution. As a result, the slurry is discharged with only a portion of the originally added STMP being a part of the slurry and an amount of precipitation being left behind in the pin mixer. Over time, the precipitation builds up in the mixer and ends up clogging the mixer. As a result, the manufacturing process needs to be shut down periodically to clean out the mixer. Such delays are costly and it is desired for them to be avoided. It is also desirable to come up with a way to reduce costs by reducing the amount of STMP needed for the manufacturing of gypsum products and to prevent precipitation from occurring.
The precipitation detrimentally affects the quality of the board that is produced because some of the precipitation left in the pin mixer can break down and be discharged through the outlet, which in turn causes paper break down and damage to the forming process which may decrease the gypsum setting time. Accordingly, it is desired to develop a method that allows the STMP to be more effectively and efficiently added to gypsum slurries in order to reduce the amount of STMP needed and to avoid the problems associated with the build up or deposits of phosphates being formed in the pin mixer.